Friday, April 24, 2015

House Creek Trail

The House Creek Trail begins at Blue Ridge Road near the McDonald's restaurant behind Crabtree Valley Mall, where it links with the Crabtree Creek Trail.  It runs south for 2.8 miles, ending at the Reedy Creek Trail at the east side of the pedestrian bridge crossing over the I-440 Beltline just north of Wade Avenue.  The trail crosses over the Beltline on Glen Eden Drive, then runs between the Beltline and Ridge Road, following the course of House Creek.  At the south end of the trail, you can turn west on the Reedy Creek Trail and ride over the Beltline to the NC Museum of Art, continuing on along Reedy Creek Road all the way to Umstead State Park.  Alternatively, you can turn east on the Reedy Creek Trail, and pass along the edge of the campus of Meredith College to Faircloth Street.  From that point, you can continue eastward along the Rocky Branch Trail.

Some of the other greenway trails, such as the Crabtree Creek and Walnut Creek Trails, run east-west, and thus cut across the trend, or strike, of the rock units in the area.  In contrast, the House Creek Trail runs generally parallel to strike, passing through schist and gneiss of the Crabtree terrane.  These metamorphic rocks are thought to have originated as explosive volcanic layers and muddy sedimentary layers.  Perhaps the most distinctive rock type in this terrane is graphite schist, a soft black metamorphic rock that occurs in several thin layers, and was once mined for the graphite.  The graphite mines are the source of the name Lead Mine Road - it was pencil lead, not the metal lead.  Construction activity along Blue Ridge Road in 2011-2013 exposed abundant graphite schist, but it is has since been covered.
Graphite schist exposed during construction along
Blue Ridge Road, Summer 2013
Location is about 100 yards south of trail.

At its north end at Blue Ridge Road, the trail passes through Marshall Memorial Park, featuring a monument to a local World War II hero.  At the memorial, there are several huge blocks of a very interesting variety of banded gneiss, containing the minerals hornblende, biotite, and garnet, as well as feldspars.  There are also some zones of granite in some of these rocks. Unfortunately, these rocks are not locally derived; they are probably from the Blue Ridge Mountains.  (If anyone reading this has information, please send a comment!)

As you continue along the trail, passing behind the new condominiums, you may see sparse exposure of weathered, light-colored schist in the creek.  Continue up the hill and you soon will see tennis courts, picnic shelters, and playground of Glen Eden Pilot Park.  The trail continues along the Glen Eden Drive overpass across the Beltline, then does a U-turn and passes through a tunnel beneath Glen Eden.  The stretch from here to Lake Boone Trail passes through thick woods.  As you go south, you have noisy traffic of the Beltline on your right and bucolic House Creek on your left.  There is not much geology to see for most of the year.  In late fall and winter, you may get a glimpse of some very beautiful stretches of the creek, with some locally excellent rock outcrops.

Two layers of the graphite schist run roughly parallel to Ridge Road, off to your left.  On the geological map figure below, you may be able to see that the thickest of these layers runs through the neighborhoods between the creek and Ridge Road.  The locations of two former mines are shown along this layer, between Glen Eden Drive and Lake Boone Trail.
Excerpt from Geologic Map of the Raleigh West quadrangle
(Blake, D.E., 2008, NC Geol. Survey Geol. Map Series 15)
Graphite schist layers are gray bands in eastern half, old mine
locations are indicated by square symbols and red numbers.

In the early 1990's, a homeowner "rediscovered" one of these mines while digging in his back yard!
Exploring a 150-year-old graphite mine in Raleigh

The graphite layers continue south, running through the campus of Meredith College, under the Method Road greenhouses of NCSU, across Kaplan Drive, Avent Ferry Road, and the Walnut Creek Trail just downstream from Lake Dam Road.

Continue south and through the tunnel beneath Lake Boone Trail.  During construction of this portion of the trail, nearly horizontal layers of rock were exposed where the large retaining wall now stands.  If you continue just a bit further, to where the trail goes downhill and bends to the right, you can see a natural ledge outcrop of schist in a small creek on the left side of the trail.  If you take a minute to examine this exposure, you will see that the schist layers are nearly horizontal, or very gently dipping.  This is different from most of the metamorphic rocks in the Raleigh area, which dip steeply, even vertical in some cases.  The reason for the shallow dip here is that this location is right along the hinge-line of a large fold, the Raleigh antiform.  The antiform is basically a huge arch of rock layers, and we are right at the top of the arch here.  This antiform can also be studied at Shelley Lake.

Nearly horizontal layers of schist along the hinge of
the Raleigh antiform

Continue just a short distance ahead and you may see some large outcrops of light-colored gneiss in House Creek below you to the right.  These were probably volcanic in origin.
Metavolcanic rock outcrop in House Creek
just east of Horton Street

From this point, continue south along the trail, up one of the steepest hills in the greenway system, and at the top you connect with the Reedy Creek Trail.

If you turn left and take the Reedy Creek Trail, you can connect with the Rocky Branch Trail, and eventually the Walnut Creek Trail, and continue all the way to the Neuse River Trail if you so desire.

Tuesday, March 24, 2015

Crabtree Creek Trail

At present, the Crabtree Creek Trail greenway runs 14.6 miles between Anderson Point Park on the Neuse River and a small parking area located at the end of Lindsay Dr. in the Oak Park neighborhood, just east of Duraleigh Road in west Raleigh.  In the future, the plan is to connect this greenway with the trail system at Umstead State Park.  This trail description begins at Lindsay Dr. and follows the trail downstream to Anderson Point Park.  The posted mile markers along the greenway measure the trail in the opposite direction, beginning at Anderson Point.

Terranes and Structure
The trail crosses two groups, or terranes, of metamorphic rocks.  The rocks of any one terrane are closely related in age and in mode and location of origin.  Terranes are separated from one other by faults.  The western portion of the trail passes through the Crabtree terrane, which includes distinctive mica-rich schist, including one variety with abundant graphite.  The trail crosses a major fold, the Raleigh antiform, where the rock layers have been folded into a huge arch-shaped structure.  To the east, the trail crosses the Nutbush Creek fault, then runs through the Raleigh terrane, consisting first of light-colored Falls leucogneiss, then mainly of dark and light-colored banded Raleigh gneiss.  In the vicinity of Wake Med Hospital, the trail passes out of the two metamorphic terranes and into the younger granite of the Rolesville batholith.

Generalized geological map showing Raleigh area and the Crabtree Creek Greenway.  Geological features are shown on the map; rock units are shown in different colors.  (Map source: Clark and others, 2004, Preliminary bedrock geologic map of the Raleigh 30' x 60' quadrangle:  NCGS OFR 2004-02)

Stream Geomorphology
As you travel the greenway, on foot or bicycle, you will be able to get a good view of several common features of most rivers and streams.  First, note the bends in the course of Crabtree Creek.  On the outside of a bend, the water moves more rapidly, and on the inside, more slowly.  Because of this difference in velocity, the stream's erosive force is concentrated on the outside.  As a result, it is common to see bedrock exposed in these locations, with a steep stream bank.  This is called a cut bank.  You may see evidence of cut bank erosion from a recent storm, as large trees toppling toward the stream as their roots have been undercut.  At the inside of a stream bend, sediment may be deposited from the more slowly moving water, so at these locations you may find a flat, sandy area, called a point bar, and a more gently sloping stream bank.  It is largely by deposition on the inside, and erosion of the outside, of these bends, that a stream gradually modifies its course.  In an extreme case, when a bend becomes so pronounced, the stream may take a shortcut during a time of high water, and cut off the bend, producing an ox-bow.  Ox-bows are much more common in old, large meandering rivers in a low-gradient area such as the Mississippi Delta, or the North Carolina Coastal Plain.

Most changes in a stream system occur during times of high and fast water.  In places, you may notice some flat areas next to Crabtree Creek that may be swampy or muddy looking, especially after rain.  These are part of the creek's floodplain, an area that may be covered with water during a flood.  During times of high water, the stream is moving rapidly and is carrying a lot of sediment with it.   As the stream overflows its banks, the water spills out toward the floodplain.  Once the water is out of the stream channel, it slows and cannot carry all of the sediment.  The coarser sediment, usually sand and silt, is deposited right next to the stream, and the finer clay is carried farther from the stream and eventually deposited in the floodplain.  You may notice low ridges along the stream bank.  These were deposited by floodwater and are called natural levees.  Along many large rivers, these are fortified, enlarged, and raised higher by humans in order to prevent flooding.

Environmental and Engineering Geology
Crabtree Creek is the major flood threat for the city of Raleigh.  Part of the reason for this situation is that through the years, much of Crabtree Creek's floodplain has been the site of much commercial, and less residential, development.  These buildings have been constructed in harm's way, and may be threatened whenever there is a major rainfall event.  Another critical factor in flood threat is the continued development and deforestation within the drainage basin of Crabtree Creek.  Trees and other plants hold the soil in place, and serve as a buffer when it rains, holding water and passing it into the ground water system, where it moves very slowly toward small creeks and eventually to Crabtree Creek.  When humans replace the vegetative buffer with impermeable rooftops and asphalt surfaces, rainwater flows rapidly into the nearest creek, or into the city's storm sewers.  The more development, the faster the floodwater rises.  Among the sites along Crabtree Creek that are highly susceptible to flooding are Crabtree Valley Mall and the shopping center at Six Forks and Old Wake Forest Roads, though there are many others.

Just south of Umstead Park at I-40, Crabtree Lake was created by damming Crabtree Creek, helping somewhat to control flooding; other smaller dams have also been built in the basin.  The U. S. Geological Survey has installed numerous stream-gauging stations along Crabtree Creek.  These allow continuous monitoring of conditions and are used to foresee flood threats and drought conditions as well.

Trail Description (from west to east)
Starting out from the Lindsay Dr. parking lot, head east along the greenway.  The first rock exposure along the trail is an impressive two-tiered cut on the left.  The sloping hillside was excavated during construction of the greenway and the parallel sewer line.  See photo.

Crabtree Creek gneiss exposed in roadcut.

A close inspection of the rock shows its light color with thin streaks of darker material.  The light-colored portion consists of feldspar and quartz, while the dark is mainly the black mica mineral, biotite.  The rock is part of the Crabtree Creek granitic gneiss, which is a metamorphosed body of granite within the Crabtree terrane.  The Crabtree terrane contains this unit and several others.  Some were originally granite, while others were originally volcanic or sedimentary rocks.  All were changed significantly by metamorphism during the construction of the Appalachian Mountains.

Biotite streaks (lineation) in granitic gneiss.

The nearly horizontal black streaks of biotite show a feature known as lineation, and attest to the stretching forces that affected these rocks.

A bit further on, the trail crosses to the south side of the creek, where a spur trail enters from Galax Drive.  Just a couple hundred feet further, there is a large cut bank exposure of Crabtree Creek gneiss along the creek to your left near the end of the wooden fence.

Cut-bank exposure of granitic gneiss.

If you scramble down to the rock, you can see a vein of quartz that cuts through the granitic gneiss.

Quartz vein in Crabtree Creek granitic gneiss.

Moving along, the trail passes beneath busy Creedmoor Road.  You can see Crabtree Valley Mall to the left, built on the floodplain of the creek.  The ground-level parking area behind the Macy's store is the first part of the mall to be affected during flooding.

Don't park here during high water!

Moving ahead, along the back of the mall, there are numerous large rocks remaining from construction activity.  These rocks differ from the granitic gneiss we have seen because they contain more mica, and some contain the black mineral hornblende as well.  If the sun shines on them, you may see a reflective sheen.  Most of these schists are metamorphosed sedimentary or volcanic rocks.

Crabtree terrane schist behind the mall.

Continuing on, you approach a McDonald's restaurant.  Before it was here, there was a prominent rock exposure that was a favorite site for geologists and geology students to puzzle over.  Sadly, corporate power (and some dynamite) spelled the end for this rock, fondly dubbed our "roche moutonee," a French geological term for a glacial feature which it resembled (though it certainly was not a glacial feature itself!).  The roche mountonee was primarily composed of tightly contorted schist rich in quartz and feldspar, but also containing garnet and other minerals.  Nearby, there are parallel rock layers of schist that is rich in the soft black mineral graphite.  During construction in the Crabtree Valley neighborhood, watch for tell-tale black soil and rock; in 2013, a thick layer of graphite schist was exposed along Blue Ridge Road just up the hill to the south of the mall.  (Incidentally, this graphite schist is the source of the name Lead Mine Road.)

McDonald's parking lot, with pieces of the old outcrop.

Crabtree Creek has not always flowed in its current course behind the mall.  Prior to around 1970, it flowed close to Glenwood Avenue.  Its path was re-routed to provide space for construction of the mall.

If there has been a recent heavy rain, look for evidence of it on the trail through this section.  You may see sand and silt accumulated on the trail or above the creek bank.  You may also see debris that was trapped in trees or structures such as bridges.

After you pass under Glenwood, the trail crosses a bridge to the north side of the creek.  Soon, you pass across the hingeline of the Raleigh antiform, a huge geological fold structure.  Layers from the west side of the hinge are repeated on the east.  See the Shelley Lake greenway blog for more detail on the Raleigh antiform.  Below is part of a detailed geological map to show this feature.

Excerpt from Geological Map of the Raleigh West quadrangle (Blake, D. E., 2005; available at <>).  Raleigh antiform runs through center with short arrows; greenways shown in green lines.  Crabtree Valley Mall is just west of center; Falls leucogneiss is at eastern edge.

Soon, a large rock outcrop is visible ahead on the left.  It forms a small cliff on the north (left) side of the creek.

Cliff outcrop

As you pass behind some apartment buildings and approach the intersection with the Mine Creek Trail, this is a good spot to see a natural levee along the creek.  If it has flooded recently, you will see newly accumulated sediment, attesting to how natural levees develop.

View looking west (creek on left) showing natural levee

Near the intersection with the Mine Creek Trail (see separate blog for this trail), a distinctive rock unit of the Crabtree terrane passes beneath the trail.  It is known as the Horse Creek schist, and is best known to geologists and rockhounds from a roadcut on Highway 98 about two miles west of Wake Forest.  This schist contains numerous garnets and other metamorphic minerals, including staurolite and kyanite.  The best examples of this rock here are the huge blocks between the apartment buildings and Mine Creek, but the garnets at Highway 98 are bigger and redder!

You cross Mine Creek on a very narrow bridge, and continue along the north side of Crabtree Creek.  You pass a spur trail to North Hills Park, then pass beneath the I-440 Beltline.  You again cross to the south side of the creek.  The next section of the greenway parallels Alleghany Drive.  For a fairly long stretch, the trail and creek continue in a fairly straight path, until they both begin to take a sharp bend to the right.  You will see huge rock cliffs on the far side of the creek.  You should begin to see them about where the power transmission line crosses overhead, or slightly before.

This is significant for a couple of reasons.  First, the hard rock that makes up the cliffs is a new rock unit, the Falls leucogneiss, and belongs to a new terrane, the Raleigh terrane.  When it encountered the hard rock, the creek could not easily cut through, and turned to parallel it for a stretch.  The rock units in this area run just a bit (about 10 degrees) to the east of north; to a geologist, this is known as their strike.  Crabtree Creek generally cuts right across the strike of the rock units, but here, it parallels the strike for a short distance before turning back to the left and continuing across the strike.  Here is also the point at which the trail crosses the Nutbush Creek fault, which separates the Crabtree and Raleigh terranes.

The Falls leucogneiss is so hard that all streams have difficulty cutting through.  Stream valleys narrow, and rapids and waterfalls are common.  As a result, where streams cross this rock was a favorite site for dams to be constructed.  In the past, these were mill dams, such as Lassiter and Yates Mills, and formerly at Falls.  The current Falls Lake Dam is also constructed on the leucogneiss, to provide water for Raleigh, and recreation opportunities.  In addition, the Falls leucogneiss was a favored building stone in the area.  Many quarries existed, and provided stone for many of the older homes, businesses, and public buildings in Raleigh.  A good example is Broughton High School.  A wall of an old quarry may be seen behind the Harris Teeter grocery store in Glenwood Village Shopping Center, at Glenwood and Oberlin.

This rock also causes the trail to end temporarily.  The pavement ends and the trail runs a bit further, unpaved and rocky, until the leucogneiss poses too much of an impediment.  To continue on the greenway, it is necessary to proceed uphill, around Aldert Root Elementary School, down to Lassiter Mill Road, where you turn left, cross the creek, then turn right to continue on the greenway.  But, before you cross the bridge, you should turn left to visit Lassiter Mill Park.  Here you can see the site of the old Lassiter Mill, the dam which is still there, and a spectacular outcrop of fresh Falls leucogneiss.

Lassiter Mill Dam and outcrop, January 2015

Same location, April 1981, with remains of mill

A plaque at the old mill site has a description of the mill and artifacts of the structure.

If you examine the rock closely, you should note an outstanding characteristic of the Falls leucogneiss:  its strong lineation.  Most metamorphic rocks exhibit some sort of parallel planes or layering, called foliation.  Instead, the leucogneiss contains stretched out lines.  In the leucogneiss, the lines are nearly horizontal and are oriented about ten degrees east of north, parallel to the strike.  The lines are made up of dark minerals, biotite (black mica) and magnetite, which makes this rock relatively magnetic.  The leucogneiss started out as a body of granite, without the lineation.  The lineation was imposed on the solid granite during later motion along the Nutbush Creek fault, which moved horizontally, stretching out and recrystallizing the minerals in the granite.

A block of fresh Falls leucogneiss, showing the strong lineation.  The pink color of the fresh rock changes to a tan or light brown upon weathering.

After carefully crossing Lassiter Mill Road, continue on the greenway trail.  Ahead, the trail again crosses the creek on a bridge.  You may notice that the floodplain is wider beginning here, in the vicinity of Rothgeb Park.  The reason is that you have crossed out of the hard Falls leucogneiss and into the more easily erodible Raleigh gneiss.  Near this spot, you may be able to see evidence of recent cut-bank erosion.

Cut-bank erosion

Just ahead, there is a large sloping rock outcrop, containing gneiss and intrusions of pegmatite, a variety of granite with large crystals.

Sloping outcrop

This large feldspar crystal is part of a small pegmatite intrusion.

At the same location, the cut on the uphill side of the trail displays these lineations, that suggest past fault movement.

You will cross the creek again, twice, returning to the south side as you approach Anderson Drive.  One of the U.S.G.S. stream monitoring stations is installed on the bridge here.  Cross Anderson and follow the road a hundred yards or so to where the trail continues alongside of the creek.  At this point, as you pass Our Lady of Lourdes Church and School, the floodplain widens drastically, extending all the way north to Six Forks Road, and south for a similar distance.  Like Crabtree Valley, the shopping centers and businesses near the intersection of Six Forks and Wake Forest Roads are highly susceptible to flooding.  In fact a number of older businesses closed down, and newer ones have been constructed on elevated fill, brought in to raise their buildings out of danger.  Of course, this practice just displaces flood waters so that areas downstream will be affected more.

You cross again to the north side of the creek, at a spur trail that leads to Joyner Park and School.  In a short distance, a sizable tributary enters Crabtree Creek from the north.  This is Big Branch, which itself poses flood threats to residential areas along its path through North Raleigh.  Large boulders are seen along the trail near here; some of them are good examples of the banded Raleigh gneiss.

Raleigh gneiss boulder near Big Branch

As you approach Wake Forest Road, you get a good impression of the wide floodplain and see more natural levees along the creek.

View looking east toward Wake Forest Road, showing natural levees

Cross Wake Forest Road, proceed ahead, and under the railroad tracks and Atlantic Avenue.  You continue through a marshy area, then climb a steep and sharply turning wooden portion of the trail.  Here, Crabtree Creek takes a sharp bend.  Looking down to the other side of the creek, you see the point bar, where sediment is deposited and the terrain is fairly flat.  The trail lies on the outside of the bend, where erosion is concentrated.  Steep rocky slopes of Raleigh gneiss are just below the trail.

View across sharp bend in Crabtree Creek from the high boardwalk

Cut-bank outcrop of Raleigh gneiss just beneath boardwalk

The best exposures of Raleigh gneiss in the area lie along Pigeon House Branch, the creek that runs along Capital Boulevard between here and downtown Raleigh.  At present, you can see spectacular outcrops in Pigeon House Branch between the Wade Avenue and Wake Forest Road turnoffs.

Raleigh gneiss exposed in Pigeon House Branch near intersection of Capital Blvd. and Wake Forest Rd. (not on the greenway)

Next, you cross under Capital Boulevard and under another set of railroad tracks, and enter a large wetlands area, which, despite its urban setting, is a haven for birders.  This section of trail is mainly a long boardwalk that ends at Raleigh Boulevard.  The trail takes a jog to the right here and continues along Crabtree Creek toward the southeast.  From here to New Bern Avenue, the floodplain continues to be wide and there are lots of low wetland areas.

In a short distance, the creek takes another sharp bend.  Although there are no cliffs here, you may see a bit of rock amid the silt and mud on the inside of the bend across the creek.

Gravel bar

Then you may notice a gravel bar near where a tributary enters from the far side.  This is likely part of a delta, where sediment is deposited as the tributary enters Crabtree.  In a short distance, at another bend, you may see a shoal of dark-colored rocks.  This is likely the location of a narrow band of unusually hard rock called diabase.

Rocky shoal, possible diabase

After cruising along on the floodplain for some distance, the trail veers to the right and climbs up the hill.  You pass a playground, then climb some more, eventually passing an old utility building and then down a steep curving stretch, returning to the creekside.  Trough this hilly portion of trail, the greenway diverges from the sewer line, which cuts straight through the rocky hills here.  After descending the hill, you will see a gate with an impressive line of boulders.  These boulders are good examples of Raleigh gneiss and granite intrusions cutting the gneiss.

Boulder of Raleigh gneiss at gate

Boulder of gneiss at gate with pegmatite intrusion containing dark crystals of hornblende

Raleigh gneiss boulder showing folded layers

Boulder of gneiss with dark layer

The trail continues ahead to Milburnie Road, where you turn left.  Along this short stretch there is a cut-bank outcrop along the creek below you, and there is a roadcut along the sidewalk across the road, exposing mixed gneiss and granite.

Roadcut along Milburnie Road sidewalk

At about the point where the trail passes under New Bern Avenue, we leave the metamorphic rocks of the Raleigh terrane and enter the younger granite of the Rolesville batholith.  This batholith is an enormous body of granite that extends from here to Zebulon, and stretches north to Henderson and south into Johnston County.  It is one of the largest granite bodies in the eastern U.S.  The trail continues through granite all the way to its end at Anderson Point.

The greenway runs behind apartment buildings for a distance, then crosses the creek on a bridge.  At this point, you may be able to see an exposure of granite on the side of the creek, and possibly another in the creek, if the water is not too high.

Creekside granite exposure

In a short distance, you will come upon several huge blocks of granite, uncovered during excavation for the new sewer line here.

Granite blocks along sewer line

You cross under the I-440 Beltline and US-64/264 Bypass interchange.  Soon, the trail again crossed the creek.  From the bridge, you may be able to see a large outcrop of granite at the side of the creek.

Granite seen from greenway bridge

Cross under New Hope Road.  Then, between Milepost 1 and the power transmission line, you will encounter several hillside outcrops of granite.

Granite outcrops on hill slope

Ahead, the creek takes a sharp bend to the left, and there is a large cut-bank outcrop.  When you get up to this exposure you will see that it is big enough the explore a bit, and you may be able to see the essential components of the granite:  feldspar, quartz, and black biotite mica.

Cut-bank granite exposure from a distance

Same outcrop close-up

Ahead, just before you pass beneath US-64/264 Bypass, there is a nice hillside granite exposure along a wooden boardwalk portion of the trail.

View of trail-side outcrop and boardwalk, looking west

Continue from here, cross Crabtree Creek on a high bridge, and climb the hill to the end of the trail at Anderson Point Park.  Here you will encounter a number of boulders of fresh granite.

Granite boulder at end of trail.  This is typical medium-grained Rolesville granite.

 This granite has gneissic streaks and a dike of coarse-grained pegmatite

From Anderson Point, you can follow the Neuse River trail south toward Clayton, or north toward Wake Forest.

This Crabtree Creek Greenway description was posted by Skip Stoddard, with assistance from Tyler Clark.